In order to connect an integrated circuit formed in an element region of a semiconductor substrate to a circuit (an external circuit) provided on the outside of the substrate, a pad electrode has been formed in a non-element region in the peripheral edge portion of the semiconductor substrate. The pad electrode is electrically connected to the integrated circuit, and furthermore, is electrically connected to the external circuit through a lead.
The connection of the pad electrode and the lead is performed through a bump (projection) electrode. This kind of bump electrode has conventionally been formed by electrolytic plating such as gold, solder and the like.
FIGS. 18A to 18E are sectional views showing steps of a method for forming a bump electrode according to the prior art. In order to form the bump electrode, first of all, a plurality of pad electrodes 83 (only one of them is shown in the drawing) made of Al are formed in a non-element region covered with an insulating film 82 in the peripheral edge portion of a semiconductor substrate 81 as shown in FIG. 18A.
More specifically, for example, an Al film is formed by using a sputtering method, and the Al film is then processed by photolithography and RIE, thereby forming the pad electrode 83. The pad electrode 83 is electrically connected to a semiconductor device constituting an integrated circuit (not shown) which is formed on the semiconductor substrate 81.
As shown in FIG. 18A, next, an interlayer dielectric film 84 is formed over the whole surface of the semiconductor substrate 81, and the interlayer dielectric film 84 provided on the pad electrode 83 is then removed selectively by etching, thereby forming an opening.
As shown in FIG. 18A, then, a Ti film 85, a Ni film 86, and a Pd film 87, which are conductive films necessary for electrolytic plating, are sequentially formed by using the sputtering method, for example, so as to cover the exposed pad electrode 83 and the interlayer dielectric film 84.
The Ti film 85 is a barrier film of a bump electrode material, the Ni film 86 is a contact film to perform a contact of the pad electrode 83 with the bump electrode, and the Pd film 87 is an oxidation inhibiting film for inhibiting the oxidation of the Ni film 86.
As shown in FIG. 18B, thereafter, a photoresist pattern 88 is formed. The photoresist pattern 88 has a thickness of about 20 μm and is provided with an opening in a region where the bump electrode is to be formed.
As shown in FIG. 18C, subsequently, the films 85 to 87 are electrically charged by a current-carrying pin to perform the electrolytic plating, for example. Thus, a bump electrode 89 made of gold or solder is selectively formed in the opening. In this case, it is necessary to previously cover, with an insulator, a region which should not be subjected to the electrolytic plating, for example, the back side of the semiconductor substrate 81.
As shown in FIG. 18D, next, the photoresist pattern 88 is taken away, and the films 85 to 87 are then subjected to wet etching by using the bump electrode 89 as a mask. Thus, the films 85 to 87 are caused to remain under the bump electrode 89, thereby insulating the bump electrodes.
As shown in FIG. 18E, finally, the bump electrode 89 is subjected to reflow by performing heating while applying a flux.
However, such a method for forming the bump electrode 89 has the following drawbacks.
First of all, the electrolytic plating is used for the formation of the bump electrode 89. The electrolytic plating requires a large number of steps. For this reason, there is a problem in that the number of steps is large.
At a wet etching step for the various films 85 to 87 which have been subjected to the sputtering prior to the plating, a wet etching step and a washing step should be performed several times according to the kind of the film, and a vast amount of water is required.
Recently, the pad electrode 83 has also been formed more finely with an increase in the fineness of the element. Therefore, there are some cases where an antireflection film such as a TiN film to prevent reflection during exposure at a lithography step is formed on an Al film acting as the pad electrode 83.
Although the TiN film is effective as a barrier metal, it has poor adhesion to the bump electrode made of metal such as solder, gold or the like. For this reason, after the Al film is processed to form the pad electrode 82, the TiN film should be removed. Consequently, the number of steps tends to be increased still more.
Moreover, a resist pattern to be used at the etching step which is an ordinary semiconductor process has a thickness of several μm, while the resist pattern 88 to be used at the plating step has a great thickness of 20 μm as described above. For this reason, there is a problem in that the photolithography step for forming the resist pattern 88 will be hard to perform in the future.
Moreover, in the case where a semiconductor device having the resist pattern 88 formed therein is immersed in a strongly acidic plating bath, the resist pattern 88 is eluted as an organic impurity into the plating bath at the electrolytic plating step so that the composition balance of a plating solution is lost.
As a result, a variation in the reflow reaction temperature of the bump electrode 89 is generated at a reflow step of the bump electrode 89 and a mounting connection step thereof. Consequently, there is a problem in that the reliability and yield of the connection is deteriorated.
By the high functionality of the element and various mounting steps, a reduction in the size of the pad electrode and an increase in the number of the pad electrodes are accelerated. Therefore, a reduction in the above-mentioned variation will be increasingly significant in the future in order to keep the reliability of the pad electrode 89.
In order to eliminate the above-mentioned drawbacks, there has been known a method in which a metallic ball such as a solder ball, a gold ball or the like is provided on an Al pad electrode and is then pressure welded and melted to form a bump electrode.
However, in the case where the solder ball is provided on the Al electrode pad, a barrier film and an adhesion layer are to be formed in order to prevent Sn constituting the solder ball from being diffused into the Al electrode pad. In this respect, the number of steps is increased in the same manner as in plating film formation.
As one of metal film forming methods, a fine particle film forming method has been known and application to a part of the method to a process has been investigated. As a method for application to a mounting technique, there has been investigated a method for forming a bump electrode made of fine particles of gold (Au) by depositing the Au fine particles on a pad electrode.
In the case of this method, it is necessary to deposit a large quantity of Au fine particles in order to form a bump electrode having a required thickness. Under the existing conditions, however, a deposition rate or the like is insufficient. Therefore, there is a problem in that such a method does not correspond to a real process. In other words, there has not been a real electrode structure using a conductive fine particle film.